Highly enriched multiply-labeled stable isotopic compounds as atmospheric tracers

ABSTRACT

COMPOUNDS MULTIPLY-LABELED WITH STABLE ISOTOPES AND HIGHLY ENRICHED IN THESE ISOTOPES ARE READILY CAPABLE OF DETECTION IN TRACER EXPERIMENTS INVOLVING HIGH DILUTIONS. THUS, FOR EXAMPLE. 13C18O2 PROVIDES A USEFUL TRACER FOR FOLLOWING ATMOSPHERIC POLLUTION PRODUCED AS A RESULT OF FOSSIL FUEL BURNING.

United States Patent US. Cl. 23-232 R Claims ABSTRACT OF THE DISCLOSURECompounds multiply-labeled with stable isotopes and highly enriched inthese isotopes are readily capable of detection in tracer experimentsinvolving high dilutions. Thus, for example, 0 0 provides a usefultracer for following atmospheric pollution produced as a result offossil fuel burning.

Background of the invention The invention described herein was made inthe course of, or under, a contract with the Us. Atomic EnergyCommission. It relates to a method of tracing labeled compounds at highdilutions and more particularly to the use of compounds multiply-labeledwith stable isotopes as tracers.

The use of labeled compounds in the determination of various chemicaland physical phenomena are well known in the art. Such compounds areusually described as tracers and until rather recently have generallybeen labeled with radioactive isotopes. The literature does disclose theuse of tracers singly labeled with a particular stable isotope. 'Iheisotopes most often used for this purpose are C, N, and 0. See, e.g.,Myerson and Fields, Mass Spectrometry and Carbon-l3 Labeling, Science166, 325-8 (Oct. 17, 1969). Heretofore, the use of stable isotopes forlabeling has been severely limited by their scarcity and cost. However,the Los Alamos Scientific Laboratory, operated by the University ofCalifornia for the US. Atomic Energy Commission, is now producing asubstantial amount of C, N, and O which are available for purchasethrough the Atomic Energy Commission.

Tracer compounds for following the spread and distribution ofatmospheric pollutants and contaminants are continually being sought.Radioactive tracers are known to be capable of performing this task.See, e.g., Nickola et al., Inert Gas Tracer System for Monitoring theReal-time History of a Dilfusing Plume or Puff, J. Ap. Meteorology 9,621-6 (1970). However, radioactive tracers are now in ill repute asatmospheric contaminants themselves. Tracer methods using stableisotopes have been applied to atmospheric pollution studies. Dequasieand Grey, American Laboratory (Dec. 19, 1970), report the use of 8 0 and0 0 for tracing the course of pollutants in the Salt Lake City area.Unfortunately, the variation in normal abundance of stable isotopes hascustomarily imposed a limitation on their use in tracer experimentsinvolving high dilutions. The problem has been that compounds singlylabeled with stable isotopes are exceedingly difficult to detect at highdilutions such as normally prevail in the atmosphere.

Summary of the invention We have now found that by using compounds withseveral labeled positions, this difiiculty may be avoided. Examples ofsuch compounds are organic compounds completely substituted with C andcarbon dioxide made Patented Jan. 29, 1974 of C and 0. Isotopes usefulfor this purpose are not limited to those given by example herein butmay be selected from the class consisting of those stable isotopeshaving an abundance less than that of the most commonly occurringisotope of the element located at each labeled position of the compound.If the compound itself is normally rare, a combination of chemicalseparationsuch as by gas chromatography-with mass spectrometry permitsdetection with very large dilutions.

Although in the process of this invention, various multiply-labeledcompounds may readily be used as atmospheric tracers, we have found thatthe triply-labeled 0 0 is particularly desirable for this purpose. Theparticular virtues of triply-labeled CO over other multiply-labeledcompounds for atmospheric tracing are that (a) it is a normal minorconstituent of the atmosphere and is thus a harmless additive; (b) CO isa chemical which is easily separated from the air for analysisisotopically; and (c) although it has been shown to be stable forseveral days, it then begins to exchange isotopically with water and istransformed to being singly labeled in the carbon atom only, while theoxygen atoms return to normal abundance. The natural disappearance ofthis triply-labeled form prevents a local region from becomingpermanently saturated with the tracer and permits sensitive detection tobe resumed after a few weeks at most. Triply-labeled CO may thereforereadily be used for monitoring the spread and dissipation of the plumefrom fossil fuel smokestacks.

The method of this invention is not limited to atmospheric tracing butreadily may be applied to any situation in which the labeled compoundwill be highly diluted. It is thus applicable to problems in hydrology,biology, and chemical process technology as well.

Concepts underlying the invention Some simple concepts of mixing asapplied to isotopic mixing and dilution problems are formulated asfollows:

A mixture of two substances A and B has a mol fraction C given by &

NA+N1B N (1) where N N and N denote mols of A, B, and total substance,respectively. A second mixture of composition C is similarly given by NCA N 2) Now let N mols of the first mixture be combined with N mols ofthe second. Its composition is C N+ CAN N +N (3) For isotope problemsinvolving high dilution, regard C as being normal abundance. Thedifference C "-C is the deviation from this value. By Eq. 3

CAI]

EXAMPLE 2 Use of depleted isotope, C =0, C =0.01, C -C "=10- Then MAL Nor 1:1000 dilution.

EXAMPLE 3 Triply-labeled CO from C and O The normal abundance of thismolecule in CO, is (0.01) (0.002) (0.002) =4 10-. Suppose thatvariations of 1:100 are detectable, so that C C =4 1O* Start with puretriply-labeled CO C '=1. Then dilutions of N'/N of 4 10- are measurablein pure CO In the atmosphere, however, where CO represents 3X10- itwould be possible to detect 12x10- of this tracer. Thus a cubickilometer of air at this concentration requires 0.1 STP liter of tracerfor detection.

DESCRIPTION OF THE PREFERRED EMBODIMENT Experiments were preformed withtriply-la-beled CO (Example 3) to determine its useful lifetime as atracer in air. The carbon dioxide-49 was prepared from C0 by thefollowing reactions:

Five milliliters of the carbon dioxide (77% mass 49) were injected intoan air-tight box, 16 cubic meters in volume, containing normal air atthe local pressure and temperature, 580 mm. Hg and C., respectively. Theair within the box was stirred with a fan for a few minutes. Samples ofthe air (6 1.) were taken before and periodically after the addition ofthe labeled carbon dioxide gas, by expanding the air from the box intoan evacuated container. All of the carbon dioxide was separated from thesample by flowing the gas first through a trap cooled to -80 to removethe water and then through a trap cooled to 190 to condense out thecarbon dioxide. The latter trap was pumped on to remove allnoncondensable gases and then the carbon dioxide was distilled into asample holder for analysis in a mass spectrometer for the intensities ofmasses 44 through 49.

4 The initial and final ratios of masses 44-49 for each run are listedin the table. The useful life-time of the carbon dioxide-49 in airdepends on the moisture content of the gas. Other experiments withmixtures of C O O and C O O in a mass spectrometer show that the oxygensexchange very slowly when the gas is dry. The half-life of the 00 -49tracer could be as long as 1300 hours under conditions as severe as run3 with the use of sensitive mass spectrometer techniques.

TABLE Related Half- Time humidity Mass-44! lite Run (hours) (percent)Mass-49 (hours) 0.15 as 7 98X10- 330 21.6 33 7 66X10- 0.05 6 61X10' 8121.8 74 5 25X10' 0.05 s 16X10' 26 5 28. 1 95 4 01X10' What we claim is:

1. A method of tracing in atmospheric applications involving highdilutions of a compound which comprises multiply labeling said compoundwith highly enriched stable isotopes selected from the class consistingof those isotopes having an abundance less than that of the mostcommonly occurring isotope of the element located at each labeledposition, and detecting said highly enriched multiply-labeled compound.

2. The method of claim 1 wherein a mixture of said highly enrichedmultiply-labeled compound with naturally occurring analogs is producedfrom a sample by chemical or physical means and said highly enrichedmultiplylabeled compound is detected within said mixture.

3. A method of tracking movements of air which comprises adding-to itcarbon dioxide having carbon highly enriched in C and having oxygenhighly enriched in O and detecting the enriched multiply-labeled carbondioxide.

4. The method of claim 3 wherein said multiply-labeled carbon dioxide istriply-labeled C 0 References Cited Kamen, Radioactive Tracers inBiology, Academic Chem. Abst., vol. 75, p. 393, No. 82326W (1971).

MORRIS O. WOLK, Primary Examiner T. W. HAGAN, Assistant Examiner U.S.Cl. X.R. 25Q303

